Photographed in Yellowstone , 3rd week of September. Looks like a winter plumage Blue-winged or Cinnamon Teal to me (?) , but the eyeline is awfully weak....
Well, you've got a teal for sure. But you have between the Blue-winged and Cinnamon ones to compare as you mention. Tough one. Do you have an image with a better head angle to get a better idea on the bill shape?
I'm really tempted to call this one a Cinnamon Teal based on thickish size of bill, more rufous neck. Could still be a Blue-winged, but to me the overall "gist" says Cinnamon. Hopefully someone else chimes in, I love these ID challenges and points that others can bring.
This is a tough one. I'm going with Cinnamon Teal. The bill looks large and spoony, the head flattens back a bit, and the neck feathers are more spotty. But I'm no expert, this could easily go one way or the other.